Organic EL displays have been developed actively in recent years. This is because an organic EL, which is a self-emissive element, is advantageous in realizing high contrast which is thought to be limited in a liquid crystal display (LCD). Further, as an organic EL element provides a high-speed response, moving images involving dynamic movements can be displayed without blurs, so that an excellent display performance can be achieved.
Currently, active-matrix type displays, in which organic EL elements are driven by thin film transistors (TFT), are becoming mainstream. These displays are fabricated by forming organic EL elements on a substrate provided with a low-temperature polysilicon TFT and the like thereon. Although a low-temperature polysilicon TFT is often used as a driving element of organic EL because it exhibits high mobility and stable operation, it involves large variations in characteristics such as a threshold and mobility. When a low-temperature polysilicon TFT is driven with a constant current in a saturated region, the brightness varies among pixels, causing a problem of non-uniform appearance on the display. As such, there has been disclosed digital drive in which a TFT is operated in a linear region and used as a switch to thereby reduce non-uniformity in display.
Further, in digital driving, as pixels are controlled by two values of whether to be lit-up or extinguished, multi-gradation can be realized by way of a plurality of sub-frames (sub-frame type digital driving) or by way of area gradation using a plurality of sub-pixels (sub-pixel type digital driving).
In the conventional digital drive of sub-frame type, a pseudo contour is easily generated, and in particular, it is difficult to suppress a pseudo contour generated by a high-speed eye movement in a still image. Further, as the screen becomes larger and has higher resolution, it has been difficult to introduce a sufficient number of sub-frames.
Furthermore, although a pseudo contour is not generated in conventional digital drive of sub-pixel type, a large number of sub-pixels cannot be introduced in a unit pixel, making it disadvantageous from the viewpoint of multi-gradation, and making it difficult to improve the picture quality.